1. Field of the Invention
The present invention relates to a structure of a MOS gate controlled thyristor (hereinafter referred to as MCT) capable of turning on/off a MOS gate by changing a polarity of voltage applied thereto.
2. Description of the Related Art
An MCT is a thyristor whose emitter and base have the same conductivity type and are short-circuited by a MOS gate type transistor when a voltage is applied to a gate electrode of the transistor. This turn-off operation requires a small amount of gate power only because the thyristor controls power consumption, and no self-turn-off operation can be performed in this thyristor. Therefore, a MOS type thyristor is known in which a control electrode is provided at a base whose conductivity type is opposite to that of the above base, and a negative bias is applied to the control electrode to discharge part of anode current as base current, thereby performing a self-turn-off operation.
FIG. 1 is a perspective view of a conventional MCT, which is disclosed in IEEE, 1991, pp. 138-141. As shown in FIG. 1, impurities are diffused into the first major surface of an N.sup.- silicon semiconductor substrate 1 to form a P-type base region 2 and a P.sup.+ -type emitter region 4, and an N-type emitter region 3 is formed in the P-type base region 2. Further, semiconductor layers are grown in sequence on the second major surface, i.e., the undersurface of the semiconductor substrate 1 to form an N.sup.+ -type layer 5 and a P.sup.+ -type layer 6 serving as a buffer region and an undersurface P.sup.+ -type emitter region, respectively. An anode electrode 10 ( A ) is formed on the undersurface P.sup.+ -type emitter region 6. On the first major surface, a gate oxide film 7 is formed so as to overlap the N.sup.- -type semiconductor substrate 1, P-type base region 2, N-type emitter region 3, and P.sup.+ -type emitter region 4. A polysilicon gate electrode 8(G) is formed on the gate oxide film 7. The gate oxide film 7 and gate electrode 8 are partially removed and opened to form a cathode electrode 9(K) in the opened area on the N-type emitter region 3 and P.sup.+ -type emitter region 4.
An operation (turn-on and turn-off) of the above conventional MCT will now be described.
First a turn-off operation will be described with reference to FIGS. 2 and 3. As shown in FIG. 2, when the anode A is positively biased and the cathode K is negatively biased, a positive voltage is applied to the gate G, an N-channel MOSFET constituted by the semiconductor substrate (N.sup.- -type base region) 1, P-type base region 2, and N-type emitter region 3 is operated to form an inversion layer 11 and inject electrons 12 from the N-type emitter region 3 into the N.sup.- -type base region 1. As shown in FIG. 3, since the electrons are injected into the N.sup.- -type base region 1, holes are injected from the undersurface P.sup.+ -type emitter region 6 into the N.sup.- -type base region 1 to vary the conductivity. The MCT is thus turned on to cause a main current 14 to flow.
Next a turn-off operation will be described with reference to FIGS. 4 and 5. When the anode A is positively biased, the cathode K is negatively biased, and the main current 14 flows, a negative bias is applied to the gate G, and a P-channel MOSFET constituted by the N.sup.- -type base region 1, P-type base region 2, and P.sup.+ -type emitter region 4 is operated to form an inversion layer 15. Therefore, the P-type base region 2 and cathode electrode 9 are short-circuited, and the P.sup.+ -type emitter region 4 and cathode electrode 9 are short-circuited, and holes 16 in the main current are discharged from these paths connecting the regions 2 and 4 and the cathode electrode 9. When the holes 16 are discharged, the electrons are prevented from flowing from the N-type emitter region 3, and the main current is stopped. The turn-off operation is thus completed. The MCT is a self turn-off element capable of the turn-on and turn-off operations described above.
However, the above-described MCT has a drawback in that the turn-off operation is difficult to perform because the MCT tends to turn on in view of its operation characteristic. At present, an improvement in turn-off characteristic is advanced. To improve the turn-off characteristic of the MCT, the concentrations of the P-type base region 2 and P.sup.+ -type emitter region 4 have to increase. In the turn-off operation shown in FIGS. 4 and 5, when the holes are discharged from the main current to the cathode electrode 9 by virtue of the inversion layer 15 formed by the operation of the P-channel MOSFET, the discharge efficiency of the holes is influenced by the sheet resistances of the P-type base region 2 and P.sup.+ -type emitter region 4. To improve the discharge efficiency, the concentrations of the P-type base region 2 and P.sup.+ -type embitter region 4 have to increase, and the sheet resistances thereof have to decrease. If, however, the turn-off characteristic has priority over the turn-on characteristic and the P-type base region 2 is increased in concentration, the turn-on characteristic is deteriorated. More specifically, in the turn-on operation shown in FIG. 2, the N-channel MOSFET is operated to form the inversion layer 11, and the electrons are injected from the N-type emitter region 3 into the N.sup.- -type base region 1 to vary the conductivity and cause the main current to flow. If the concentration of the P-type base region is increased, the threshold voltage of the N-channel MOSFET is increased and the on-voltage is also increased, resulting in a great loss in the turn-on operation.
As described above, the turn-on and turn-off characteristics of the MCT correlate with each other and, if one of the characteristics is improved, the other is deteriorated, which causes drawbacks wherein a trade-off is difficult between the turn-on and turn-off characteristics and the turn-off characteristic is difficult to improve. The conventional MCT is disclosed in Published Unexamined Japanese Patent Application No. 63-310171. According to this publication, a conventional five-layer structure is changed to a four-layer structure of pnpn to simplify a manufacturing process, and the short-circuit resistance of an emitter is lowered to perform a high-speed turn-off operation, but the drawback of difficulty in the trade-off between the turn-on and turn-off characteristics cannot be eliminated.